U.S. patent number 7,287,612 [Application Number 10/786,985] was granted by the patent office on 2007-10-30 for drive system for vehicles.
This patent grant is currently assigned to Deere & Company. Invention is credited to Fritz Glaser, Bernd Kneer, Nicolai Tarasinski.
United States Patent |
7,287,612 |
Tarasinski , et al. |
October 30, 2007 |
Drive system for vehicles
Abstract
A drive system for vehicles, especially for commercial vehicles
such as agricultural or industrial tractors, the vehicle having at
least one first wheel that is driven by an associated axle or
single-wheel drive motor and at least one second wheel, in the
drive train of which a gearbox that can be shifted between at least
two speed transmission steps is arranged. To avoid vehicle speed
drops while shifting the change-speed gearbox and to avoid
interfering shifting jolts, it is suggested to provide a device for
the detection of a shift command and a control unit, which in the
presence of a shift command automatically applies a greater load at
least on the drive motor driving the first wheel, controls the
shifting operation of the change-speed gearbox of the second wheel
and then lowers the load of the drive motor driving the first
wheel.
Inventors: |
Tarasinski; Nicolai
(Frankenthal, DE), Glaser; Fritz (Zweibrucken,
DE), Kneer; Bernd (Viernheim, DE) |
Assignee: |
Deere & Company (Moline,
IL)
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Family
ID: |
32864456 |
Appl.
No.: |
10/786,985 |
Filed: |
February 25, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040200648 A1 |
Oct 14, 2004 |
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Foreign Application Priority Data
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Apr 11, 2003 [DE] |
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103 16 862 |
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Current U.S.
Class: |
180/65.7 |
Current CPC
Class: |
B60K
6/365 (20130101); B60L 7/08 (20130101); B60L
7/18 (20130101); B60W 10/08 (20130101); B60L
3/104 (20130101); B60W 30/19 (20130101); B60K
17/043 (20130101); B60L 15/2009 (20130101); B60K
6/52 (20130101); B60L 15/20 (20130101); B60K
17/356 (20130101); B60W 20/30 (20130101); B60L
3/08 (20130101); B60K 7/0007 (20130101); B60W
10/10 (20130101); B60L 3/0061 (20130101); B60K
6/46 (20130101); B60L 7/26 (20130101); B60K
2007/0092 (20130101); B60K 17/046 (20130101); B60L
2250/16 (20130101); Y02T 10/62 (20130101); B60L
2240/463 (20130101); B60K 17/08 (20130101); B60L
2240/461 (20130101); B60L 2240/486 (20130101); B60L
2250/10 (20130101); B60L 2240/421 (20130101); B60L
2240/12 (20130101); B60L 2240/14 (20130101); B60L
2240/423 (20130101); B60L 2250/24 (20130101); B60K
2007/0038 (20130101); B60K 7/0015 (20130101); B60W
20/00 (20130101); F16H 2061/0425 (20130101); Y02T
10/72 (20130101); Y02T 10/64 (20130101); Y02T
10/70 (20130101); B60L 2240/465 (20130101); B60L
2200/40 (20130101); B60L 2240/425 (20130101); B60L
2250/26 (20130101); Y02P 90/60 (20151101); B60W
2300/152 (20130101); B60L 2240/507 (20130101); B60K
1/02 (20130101); B60L 2220/46 (20130101) |
Current International
Class: |
B60K
1/00 (20060101) |
Field of
Search: |
;180/65.1,65.3,65.4,65.5,65.6,65.7,233,247,245,246,292,293,343,371,374 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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196 15 742 |
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May 1997 |
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DE |
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197 23 776 |
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Dec 1997 |
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DE |
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197 23 776 A 1 |
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Dec 1997 |
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DE |
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197 24 681 A 1 |
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Feb 1998 |
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DE |
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199 19 454 |
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Nov 2000 |
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DE |
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199 54 544 |
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Aug 2001 |
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DE |
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101 26 348 |
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Jan 2002 |
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DE |
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0 812 720 |
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Jun 1997 |
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EP |
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1 077 150 |
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Feb 2001 |
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EP |
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2 346 124 |
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Aug 2000 |
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GB |
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Other References
Patent Abstracts of Japan, Mar. 5, 2001. cited by other.
|
Primary Examiner: Shriver; J. Allen
Assistant Examiner: Avery; Bridget
Claims
The embodiments of the invention in wich an exclusive property of
privilege is claimed are defined as follows:
1. A drive system for a vehicle, the vehicle having at least one
first wheel that is driven by a first electric machine and at least
one second wheel driven by a second electric machine, in a drive
train of which a shiftable transmission that can be shifted between
at least two speed transmission steps is arranged between the
second electric machine and the second wheel, comprising: a device
for the detection of a shift command; and, a control unit, which in
the presence of a shift command automatically applies a greater
load on the electric machine driving the first wheel, controls the
shifting operation of the shiftable transmission of the second
wheel and then lowers the load of the electric machine driving the
first wheel, wherein a generator that is driven by an internal
combustion engine serves as an energy source for the electric
machines, the electric machines being designed such that the
electric machines can be operated both as an electric motors
driving the associated wheels and as generators braking the wheels,
and wherein at least one converter and an intermediate circuit are
arranged downstream from the generator in such a way that the
generator can be driven electrically by the power released by the
electric machine operating as a generator in the electric braking
operation and thus operate as an electric motor.
2. A drive system according to claim 1 wherein at least two vehicle
axles with at least two wheels, respectively, are provided, and
wherein the wheels of at least one first axle are driven by an
electric machine and the wheels of at least one second axle are
driven through at least one shiftable transmission by one of a
vehicle drive train, an axle electric machine or single-wheel
electric machines.
3. A drive system according to claim 1, wherein at least one first
wheel through a first shiftable transmission is driven by a first
single-wheel electric machine and at least one second wheel through
a second shiftable transmission is driven by a second single-wheel
electric machine, and that the control device in the presence of a
shift command initially performs a shift of the first transmission
while raising the load of the second single-wheel electric machine
and subsequently performs a shift of the second transmission while
raising the load of the first single-wheel electric machine.
4. A drive system according to claim 1, wherein a clutch is
arranged in a drive train comprising a change-speed gearbox.
5. A drive system according to claim 1 wherein at least one
electric energy storage unit that can be mounted on the vehicle
serves as an energy source for the electric machine.
6. A drive system according to claim 1, wherein the shiftable
transmission comprises one of a planetary gearbox or a standard
transmission.
7. A drive system according to claim 1, wherein at least one
speed-reducing final drive transmission is arranged downstream from
one of the electric machines.
8. A drive system according to claim 1, wherein the electric
machine is arranged within a wheel rim.
9. A drive system according to claim 1, wherein the shiftable
transmission is arranged within a wheel rim.
10. A drive system according to claim 7, wherein the final drive
transmission is arranged within a wheel rim.
Description
FIELD OF THE INVENTION
The present invention relates generally to a drive system for
vehicles, especially for commercial vehicles such as agricultural
or industrial tractors, comprising at least one first wheel that is
driven by an associated axle or single-wheel drive motor and at
least one second wheel, in the drive train of which a gearbox that
can be switched between two speed transmission or multiplication
steps is arranged.
BACKGROUND OF THE INVENTION
For matching the drive power to different driving requirements,
vehicle drive systems have been suggested in which a separate drive
is assigned to each vehicle axle or each wheel of the vehicle.
EP-A-0 812 720 for example describes a vehicle with a drive system
of the aforementioned kind. A generator driven by an internal
combustion engine supplies the electric energy for electric motors.
The wheels of the front axle of the vehicle are driven by an
associated electric motor, while the rear wheels are driven
electro-mechanically in that the internal combustion engine drives
a drive shaft that is allocated to the rear axle of the vehicle.
Each wheel of the rear axle is assigned a summing gear and an
electric motor. The summing gear combines the drive power of the
drive shaft and the corresponding electric motor and passes it on
to the appropriate rear wheel. A change-speed gearbox that can be
shifted between at least two transmission ratios is arranged
between the internal combustion engine and the two summing gears.
When switching the change-speed gearbox, the tractive power to the
rear wheels is interrupted. This can lead to a slowing of the
vehicle and express itself in an unpleasant manner for the operator
in the form of a shifting jolt. It has been found that even when
using a change-speed gearbox that can be shifted under load the
aforementioned disadvantages cannot be avoided.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the invention to
provide a drive system of the aforementioned kind such that the
problems described above are overcome.
Another object of the invention is the provision of a drive system
of the aforementioned kind wherein vehicle speed decreases and
interfering shifting jolts are avoided when shifting a change-speed
gearbox.
In general, a drive system for a vehicle, especially for a
commercial vehicle such as an agricultural or industrial tractor,
the vehicle having at least one first wheel that is driven by an
associated axle or single-wheel drive motor and at least one second
wheel, in the drive train of which a gearbox that can be shifted
between at least two speed transmission steps is arranged. To avoid
vehicle speed drops while shifting the change-speed gearbox and to
avoid interfering shifting jolts, it is suggested to provide a
device for the detection of a shift command and a control unit,
which in the presence of a shift command automatically applies a
greater load at least on the drive motor driving the first wheel,
controls the shifting operation of the change-speed gearbox of the
second wheel and then lowers the load of the drive motor driving
the first wheel.
The drive system comprises at least a first wheel that is driven by
an associated axle or single-wheel drive motor and at least one
second wheel, in the drive train of which a gearbox that can be
shifted between at least two speed transmission stages is arranged.
Pursuant to the invention a device for the detection of a shift
command as well as a control unit are provided. The control unit
reacts to a shift command by automatically applying a greater load
at least on the drive motor driving the first wheel so that the
first wheel yields greater torque. In a time-related connection the
shift of the change-speed gearbox of the second wheel is
controlled. In the final phase of the shifting process, the load of
the drive motor that drives the first wheel is lowered again. Said
motor can then generate for example again its original torque. With
this shifting method no significant interruption or decrease in the
tractive force occurs during the shifting process, which due to the
tractive resistance could lead to the braking and deceleration of
the vehicle. Rather, the decrease in power on the wheel that is to
be shifted is compensated by a corresponding increase in tractive
force on the wheel that is not shifted and continues to drive. The
vehicle thus maintains its speed and unpleasant shifting jolts are
avoided or at least weakened considerably.
This shifting and drive strategy pursuant to the invention can be
applied in various vehicle drive concepts in a beneficial manner.
The inventive drive concept for example can be applied in a vehicle
where the rear wheels are driven in the conventional manner by an
internal combustion engine and a mechanical gearbox that can be
switched between various transmission ratios or a
hydrostatic-mechanical gearbox. The front wheels by contrast are
driven by separate drive motors, wherein a drive motor can drive
the two wheels of the front axle jointly (axle drive motor) or each
front wheel comprises a separate drive motor (single-wheel drive
motor). The drive motors preferably are electric machines,
especially electric motors. When shifting a gearbox, the drive
motor is or the drive motors are driven with increased power so
that they compensate the interruption or reduction in tractive
power occurring on the rear wheels, and the vehicle can travel with
unchanged speed.
In another beneficial drive concept, both the rear wheels and the
front wheels are driven by axle drive motors or by single-wheel
drive motors. With regard to the arrangement of shiftable
gearboxes, several possibilities arise. Change-speed gearboxes can
be arranged between drive motors of the front axle and the
associated front wheels or between drive motors of the rear axle
and the associated rear wheels or both between the drive motors of
the front axle and the associated front wheels and also between the
drive motors of the rear axle and the associated rear wheels. When
shifting the change-speed gearbox or the change-speed gearboxes of
an axle, the drive motor or the drive motors of the other axle will
experience a greater load, i.e. will be operated at higher power
consumption.
If to each of the four wheels of a vehicle a separate drive motor
with a subsequent shiftable gearbox is assigned, with the presence
of a shift command initially the change-speed gearboxes of a first
axle can be shifted between two transmission steps and during the
shifting operation the drive motors of the second axle can be
operated with increased drive power. Subsequently the change-speed
gearboxes of the second axle are shifted and during the shifting
operation the drive motors of the first axle are operated with
increased power.
The shifting operation can beneficially be conducted as follows:
Initially the torque that is required for driving the vehicle is
split among all vehicle wheels. When for example during
acceleration or deceleration of the vehicle its speed approaches a
value that necessitates a shift into a higher or lower gear, the
operator or an automatic shifting control device issues a shift
command to the control device. In preparation of the shifting
operation of the vehicle wheels of a first vehicle axle, the
control device initiates a shift of the required torque to the
vehicle wheels of the second vehicle axle by temporarily applying a
greater load on the drive motors of the second vehicle axle, while
the drive systems that are affected by the shifting operation
become torque-free so that the change-speed gearbox can be shifted.
Upon a completed shift, the necessary torque is shifted to the
vehicle wheels that have already been shifted so that the wheels
that have not been shifted yet become torque-free and can be
shifted. After shifting the change-speed gearboxes of all four
wheels, the torque that is required for driving the vehicle is
again split among all four wheels.
In a vehicle with separate drive motors and change-speed gearboxes
for each wheel, a control device can also be beneficial, with which
initially the shiftable gearboxes of two first diagonally opposing
wheels are shifted simultaneously with the existence of a shift
command, while the drive motors of the two other second wheels
experience a greater load and are shifted simultaneously by the
immediately following gearboxes that are associated with the two
diagonally opposing wheels, while the drive motors of the two first
wheels experience a greater load. In general, however, an
axle-specific shifting operation is preferable for driving
stability reasons.
With farming tractors, the change-speed gearbox for example can
comprise a first gear range that permits driving speeds of e.g. up
to 27 km/h and is used primarily during working operations. A
second gear range permits driving speeds e.g. up to 65 km/h and is
used when driving on roads. With such large jumps in gear ratios
(1:2.4) normally interfering shifting jolts occur if during
acceleration or deceleration of the vehicle a gear or range shift
is performed by the operator or by an automatic gear change system.
The shifting jolts are unpleasant especially with automatic
transmissions because here they occur unexpectedly. These shifting
jolts can be avoided by using a drive system pursuant to the
invention.
Powershift transmissions can be used as change-speed gearboxes. It
is also advantageous to use a standard transmission in the drive
train of which a clutch is arranged for interrupting the flow of
power.
A beneficial embodiment of the invention that should be
particularly emphasized provides for electric machines or hydraulic
motors to be installed as the axle or single-wheel drive motors.
Electric or hydraulic single-wheel drive motors are described in
the following also as wheel motors. Especially electric machines
offer the advantage that they can be heavily overloaded for a short
time without being damaged. The permissible consumed power and/or
the maximum torque supplied can for example be increased briefly by
a factor of 2. It is thus possible to perform the inventive
shifting operation even when a maximum drive power is demanded from
the vehicle. Even during such operations, a short increase in the
drive power of the respectively affected electric machine is
permissible. For example during a shifting process on one axle in a
vehicle with four single-wheel drives the wheel motors of the other
axle can assume, in addition to their existing tractive power, a
portion of the tractive power or even the entire tractive power of
the currently shifted axle (which can be free from drive power
during the shifting operation).
The energy source for the electric machines is preferably an
electric energy storage unit that can be mounted on the vehicle
and/or a generator that is driven by an internal combustion
engine.
Preferably the electric machine and its selection are designed such
that the electric machine can be operated both as an electric
motor, which drives the associated wheel, or also as a generator,
which slows the associated wheel down.
When using a generator that supplies the electric energy for the
electric machines, preferably a converter and an intermediate
circuit are arranged downstream from it, permitting also a
motor-driven operation of the generator. This way it is possible to
support the braking operation of the vehicle electrically by
operating the axle or single-wheel drive motors with the generator.
The electric energy they create is fed to the generator, which now
functions like an electric motor and increases the speed of the
internal combustion engine and thus supplies it with energy that
decelerates the vehicle as braking energy ("electric brake").
As a supplemental or alternative "electric brake" also braking
resistance systems can be used, which destroy the electric energy
created by the electric machines.
A preferred embodiment of the invention provides that the shiftable
gearbox comprises a planetary gearbox, particularly a powershift
transmission or a standard transmission. It is also beneficial to
arrange at least one speed-reducing final drive transmission,
especially a planetary transmission, downstream from the axle or
the single-wheel drive motor. This way the drive motors can be
operated in favorable speed ranges.
For a compact design it is advantageous to arrange the single-wheel
drive motors within the wheel rim of the associated wheel.
Likewise, a change-speed gearbox that is arranged upstream from the
drive motor and/or a wheel brake and/or a planetary gear-reducing
step that is arranged downstream from the drive motor can be
arranged within the wheel rim or in the vicinity of the wheel
rims.
To acquaint persons skilled in the art most closely related to the
present invention, one preferred embodiment of the invention that
illustrates the best mode now contemplated for putting the
invention into practice is described herein by and with reference
to, the annexed drawings that form a part of the specification. The
exemplary embodiment is described in detail without attempting to
show all of the various forms and modifications in which the
invention might be embodied. As such, the embodiment shown and
described herein is illustrative, and as will become apparent to
those skilled in the art, can be modified in numerous ways within
the spirit and scope of the invention--the invention being measured
by the appended claims and not by the details of the
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
For a complete understanding of the objects, techniques, and
structure of the invention reference should be made to the
following detailed description and accompanying drawings,
wherein:
FIG. 1 is a diagrammatic view of a vehicle drive system; and,
FIG. 2 is shifting arrangement for the electric components of a
drive system pursuant to FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the figures equivalent parts and components were assigned the
same reference numbers. The vehicle drive system shown in FIG. 1 is
provided for an agricultural tractor and comprises a front axle 10
with front wheels 12 and a rear axle 14 with rear wheels 16.
Each wheel 12 of the front axle 10 is driven by an associated
electric motor 18. The output shaft 20 of each electric motor 18 is
connected to the input shaft of a planetary gear-reducing
transmission 22, which provides its output power to the front wheel
12 via a drive shaft 24, which comprises a cardan joint, as well as
a planetary gear-reducing step 26. A mechanically actuated wheel
brake 28 is integrated into the drive shaft 24. The cardan joints
enable the steering of the front wheels 12. A steering-angle sensor
30 detects the steering angle of the front wheels 12.
Each wheel 16 of the rear axle 14 is driven by an associated
electric motor 32. Between the electric motor 32 and a change-speed
gearbox 34 that can be shifted between two transmission ratios a
clutch 36 is arranged. The output speed of the change-speed gearbox
34 is further reduced in a planetary gear-reducing step 38 and
supplied to the associated rear wheel 16. Here as well a
mechanically actuated wheel brake 42 is integrated into the drive
shaft 40, which runs between the change-speed gearbox 34 and the
planetary gear-reducing step 38.
Via a drive shaft 46, an internal combustion engine 44 drives a
generator 48, which supplies the electric power for the electric
motors 18, 32. The vehicle speed is detected by a radar sensor
50.
As can be seen in FIG. 2, the generator 48 is connected with a
frequency converter 54, which forms a generator intermediate
circuit, via a cable 52. A direct current intermediate circuit 58
with an energy storage unit that is not shown in detail is arranged
downstream from the frequency converter 54. The direct current
intermediate circuit 58 supplies via additional cables 60 the
frequency converters 62, which are assigned to the individual
electric motors 18, 32 and supply them with electric energy. The
direct current intermediate circuit 58 moreover is connected by
means of another cable 64 with a braking resistance disk 66, to
which one or more cooled braking resistance units 68 are
connected.
An electric control unit 70, designed as a micro-controller, is
provided, which is connected to a BUS system 72. Additionally the
steering-angle sensor 30, the radar sensor 50 and a shift detection
device 73 are connected to the BUS system 72 so that the signals
can be transmitted to the control unit 70 and be processed there.
The shift detection device 73 can also be a switch, which is not
shown and could be actuated by the operator, or a control unit,
which generates a shift signal automatically based on the driving
conditions.
A plurality of additional electric sensors and input devices (not
shown) can be connected to the BUS system 72. For example the
electric signals supplied by speed sensors 74 detecting the speed
of the electric motors 18, 32 as well as by speed sensors detecting
the wheel speeds, by temperature sensors recording the temperature
of the electric motors 18, 32, by position sensors detecting the
gas pedal position and the brake pedal position, and by gear
shifting sensors detecting the shifting signals for the planetary
shift transmissions 34 can be fed into the BUS system 72 so that
these signals as well can be recorded by the control unit 70 and
processed.
Moreover also an input device (not shown) can be provided, with
which the control unit can be programmed and which enables the
input of vehicle-specific data such as wheel base, track width,
diameters of the front and rear wheels, gear ratios of the
transmissions, maximum permissible speeds for transmissions and
electric motors and the like.
The BUS system is connected via a BUS system cable 76 with a
micro-controller 78 for the generator intermediate circuit 54, with
the direct current intermediate circuit 58, with the
micro-controllers 80 for the frequency converters 62 of the
electric motors 18, 32 and with the braking resistance disk 66 so
that these can be selected by the control unit 70. By means of the
BUS system cables 76, the micro-controllers 78, 80 and the direct
current intermediate circuit 58 feed electric data with respect to
current, potential and frequency to the control unit, which enables
the calculation of torque, power and the like. The control unit 70
also supplies electric control signals to the clutches 36 via the
BUS system 72, which is not depicted in more detail.
The drive system allows a vehicle in normal operation to be driven
either by all four electric motors 18, 32 that are supplied by the
generator with electric energy or solely by the two electric motors
32 of the rear axle 14. If the vehicle is accelerated or
decelerated such that a gear change is required on the change-speed
gearboxes 34 so as not to operate the electric motors with too high
or too low a speed, then the power of the electric motors 32 of the
rear axle 14 is reduced and the clutches 36 are opened with
electric signals. Now a switch of the planetary switching
transmission 34 can occur. Subsequently the clutches are again
closed by corresponding electric control signals, and the electric
power of the electric motors 32 of the rear axle 14 is again
increased.
In order to avoid an interruption in the tractive force during this
shifting process, the two electric motors 18 of the front axle 10
are selected simultaneously for the purpose of generating torque on
the front wheels 12, which will balance the drop in tractive power
occurring on the rear wheels 16. The control unit 70 hereby can
synchronize the selection of the electric motors 18, 32 such that
when the power of the electric motors 32 of the rear axle 16 is
reduced the power of the electric motors 18 of the front axle 12 is
accordingly increasingly. After shifting the planetary shifting
transmission 34 and closing the clutch 36, the power of the
electric motors 32 of the rear axle 14 is increased again and the
power of the electric motors 12 of the front axle 10 is lowered in
the same degree.
In the case of a tractor with good ballast balance, about 30% of
the available tractive power is transmitted via the front wheels
(15% per front wheel) and about 70% via the rear wheels (35% per
rear wheel) for nominal tractive power. If the power of the tractor
is not utilized fully for towing because e.g. the maximum possible
tractive power is not being run, the tractive power remains split
between the front wheels and the rear wheels roughly at 30% to 70%.
Accordingly also the lower overall drive power of the electric
motors is divided between front and back. A subsequent adjustment
of the torque on the individual wheels occurs in such a way that,
to the extent possible, roughly equal slip values are created for
all 4 wheels. This way different wheel loads and different friction
coefficients between the tires and the ground are taken into
consideration optimally. Each wheel hereby maintains the greatest
possible lateral stability force. This improves driving stability
considerably and hence driving safety. The vehicle does not veer
from the desired path.
To find out the exact slippage, a radar sensor 50 can be used to
detect the actual travel speed v. The slippage s can be calculated
from the wheel circumferential speed u and the actual travel speed
v: s=(u-v)/u.
In practice, however, it is not absolutely necessary to know the
actual and exact slippage value s for each wheel. When the speeds
of the wheels while driving straight ahead and driving in curves
correspond to the rolling condition according to Ackermann, equal
slippage exists on all wheels. According to Ackermann's condition,
the wheels of a vehicle travel on circular tracks around a common
center. From the vehicle geometry we know the wheel base, the track
width of the axles and the scrub radius. According to Ackermann,
when traveling in curves the desired speeds and desired rpm values
of the individual wheels can be exactly calculated with the
electronic control unit 70 based on the curve paths, which can be
calculated, of the individual wheels. The current steering angle
that is required is determined with the steering-angle sensor 30.
When traveling straight ahead, the wheels should have the same
circumferential speed. This likewise guarantees the same slippage
on all wheels.
Within the respective gears of the change-speed gearboxes 22, 34,
the travel speed is adjusted via the speeds of the electric motors
18, 32. The necessary torque is adjusted such that no distortion
arises between the individual wheels 12, 16. This has been
accomplished when all wheels 12, 16 have the same slippage. In a
driven wheel 12, 16 on which greater slippage is detected than the
average of all 4 wheels, the control unit 70 lowers the driving
torque. When the slippage of a driven wheel 12, 16 is lower than
the average of all 4 wheels, the driving torque is increased. This
way all 4 wheels arrive at the same slippage. This method also
enables an unproblematic driving operation when the ground is such
that different coefficients of adhesion or coefficients of drive
power result between the tires and the ground. This way it is
possible to pull all wheels evenly in accordance with their wheel
load and the ground friction coefficients and that all wheels end
up with the greatest possible lateral stability force. A wheel with
high slippage loses a large portion of the possible lateral
stability force. In extreme cases too great a slippage of
individual wheels can lead to a skidding of the vehicle from the
track; it is therefore important to control the even and low
slippage of all wheels. This task of monitoring the slip values of
the individual wheels is performed by the control unit 70, which is
part of the drive control system of the vehicle. With slight
braking that occurs only by means of the electric motors 18, 32 of
the individual wheel drives the braking torque values are also
adjusted in an analog fashion to the same negative slippage.
Tractors travel on solid roads and on soft agricultural ground.
Accordingly, this results in different tractive forces that are to
be transmitted and thus in different torque values on the
individual wheels 12, 16. The torque can be determined indirectly.
The driver specifies a desired speed. The vehicle motor 44 must
overcome road resistance and the additionally desired drive power
(e.g. on a power take-off shaft that is not shown). This results in
the drive power required by the vehicle motor 44. The drive power
values of the individual electric motors 18, 32 and/or their torque
values are divided in accordance with the specified speed. 30% of
the drive power for the front axle 10 means 15% for one wheel motor
18 of the front axle 10. Accordingly 70% of the drive power are
divided for the rear axle 14, meaning 35% for one wheel motor 32 of
the rear axle 14. The electric motor 18, 32 are also referred to as
wheel motors here.
If the road enables good transmission of the tractive force between
the tires and the ground, slippage between tires and ground should
remain below about 5%. However, with every tractive power generated
by the tractor a certain amount of slippage arises between the
tires and the ground. The control unit 70 monitors the individual
wheel speeds either by means of wheel speed sensors 74 on the wheel
motors 18, 32 or it determines them based on the electric data of
the electric motors 18, 32. It limits deviations in the slip values
of the individual wheels 12, 16 to a permissible amount. Any slip
value should not deviate more than e.g. a maximum of 5% from its
desired value. A desired slip value of 5% should therefore be
within the limits of 4.75% and 5.25%. Splitting of the drive power
and selection of the wheel motors 18, 32 are performed by the
electronic control unit 70. It forwards the necessary information
with regard to the availability of current flow, potential and
frequency to the frequency converters 62 that are assigned to the
wheel motors 18, 32 for the purpose of fulfilling the required
slippage conditions.
With a steering angle of the steering axle 10, the required drive
rpm values of the wheels 12, 16 can be established based on the
Ackermann condition. When performing a calculation with the
Ackermann condition, it can be predicted based on the geometric
rolling circumference values on the individual circular tracks what
speed rpm values as a function of the steering angle are required.
When traveling in curves the front wheels 12 drive on a larger
circle than the rear wheels 16 and must accordingly be driven with
an adjusted higher speed than when traveling straight ahead. The
Ackermann condition provides the necessary driving rpm value for
each wheel 12, 16.
If due to the quality of the road the tractive force between the
tires and the ground is no longer transmitted well, the slip value
between tires and ground can exceed a value of for example 5%. The
control unit 70 assumes the task of limiting deviations in the slip
values of the individual wheels 12, 16 to a permissible amount.
The required wheel torque is obtained as a product from the
tractive power of each wheel and its rolling radius. By means of
the adjusted transmission ratios, the torque of each electric motor
18, 32 can be determined. Torque and required wheel speed result in
the drive power of each wheel motor 18, 32.
The desired travel speed results in the wheel speed required for
it. From the driving and acceleration resistance the torque values
of the electric motors 18, 32 can be calculated. Torque and speed,
respectively, result in the required drive power values. The
overall required drive power is split among the 4 wheels in
accordance with the specified power distribution of 15% for each
front wheel and 35% for each rear wheel. Subsequent adjustment of
the torque values for each wheel drive occurs in accordance with
the specification of same slippage values for all wheels 12, 16,
i.e. subsequent adjustment of the wheel speeds and/or speeds of the
electric motors 18, 32 in accordance with the Ackermann condition.
The control unit 70 assumes this function. The wheel load
distribution can change drastically in some applications, e.g. with
a fully loaded front-end loader shovel and when backing up a steep
slope. In this case, a relatively higher power is demanded from the
electric motors 18 of the front wheels 12. To this end, generally a
brief overload of these electric motors 18 due to a higher release
power can be permitted to the extent that the driver desires it.
Due to the subsequent adjustment to the same slippage on all wheels
12, 16 no distortion in the drive occurs and the greatest possible
lateral stability force is maintained. A high lateral stability
force is important in the case of slippery ground conditions and
locations on slopes to prevent the vehicle from skidding off the
travel path.
A wheel is torque-free when no tension and no current is generated
by the converter 62 and forwarded to the wheel motor 18, 32 and
when the wheel 12, 16 is not used to drive the electric motor 18,
32 (generator operation), i.e. when electric power is neither fed
to the electric motor 18, 32 nor obtained from it. A relatively low
torque due to frictional forces caused by bearing friction and gear
friction losses, however, can still be present.
To protect the wheel drives from overload, temperature sensors are
provided in the electric components (electric motors). They feed
temperature signals to the control unit 70. In case of
impermissible heating of the electric motors 18, 32, the applied
tension and current is lowered to a permissible amount with the
help of the control unit 70. Generally these current and tension
values correspond to those for a maximum permissible permanent
load. Thus an impermissible increase in temperature normally leads
to a decrease in the vehicle's travel speed, however even with a
particularly high overload it generally does not lead to a stopping
of the vehicle. The entire behavior of the vehicle is designed such
that the highest required tractive force values in accordance with
the state of the art are also achieved with the drive system
pursuant to the invention.
Even if, when traveling downhill at the highest speed, the operator
initiates further acceleration by actuating the gas pedal, the
control unit 70 automatically lowers the drive power down to an
automatic braking operation via the four electric motors 18, 32,
which then operate as generators. The excess power is supplied to
the generator 48, then operating as a motor, for driving the
internal combustion engine 44 until it has reached its maximum
permissible speed. Further excess power can be destroyed in braking
resistance units 68 and/or be stored in the vehicle battery, if
necessary. This prevents an impermissible overspeed of the
individual wheel motors 18, 32 effectively. Moreover the operator
can be made aware of overspeed situations with suitable acoustic or
visual warning signals.
Thus it can be seen that the objects of the invention have been
satisfied by the structure presented above. While in accordance
with the patent statutes, only the best mode and preferred
embodiment of the invention has been presented and described in
detail, it is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Obvious modifications or
variations are possible in light of the above teachings. The
embodiment was chosen and described to provide the best
illustration of the principles of the invention and its practical
application to thereby enable one of ordinary skill in the art to
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. All
such modifications and variations are within the scope of the
invention as determined by the appended claims when interpreted in
accordance with the breadth to which they are fairly and legally
entitled.
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